The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
A typical automatic transmission includes a hydraulic control system that is employed to provide cooling and lubrication to components within the transmission and to actuate a plurality of torque transmitting devices such as clutches and brakes. The hydraulic control system typically includes a sump located at a bottom of the transmission that collects hydraulic oil or hydraulic fluid passed through the hydraulic control system. The sump stores the hydraulic fluid to be suctioned back into the hydraulic control system by a pump. A minimum level of hydraulic fluid is required in the sump in order to feed the hydraulic control system for all ranges of transmission operation and to account for dynamic movement of the hydraulic fluid within the sump. It is desirable to keep the amount of hydraulic fluid stored in the sump at this minimum level because excessive hydraulic fluid in the sump can interfere with rotating components of the transmission. The rotating components include for example gears, clutch plates, and interconnecting members. The rotating components traveling through the sump hydraulic fluid can generate undesirable aeration of the hydraulic fluid and increased drag, thus increasing spin losses which in turn decrease the efficiency of the transmission.
The minimum level of hydraulic fluid that must be stored in the sump varies based on factors including hydraulic fluid operating temperature, pressure, and vehicle speed. It is desirable to store excess hydraulic fluid out of the sump and in a separate area that does not interfere with rotating components. One solution is to transfer hydraulic fluid using a passive thermally actuated valve between the sump and a storage area such as a front or side cover of the transmission. Known passive thermal valves and storage area chambers have overflow ports allowing hydraulic fluid to flow between the sump and the storage area based primarily on the temperature of the hydraulic fluid.
For most operating conditions, oil level in the storage area will remain at the level of an oil pipe connected to the sump. At some extreme operating conditions, however, such as high vehicle speeds which generate high oil temperature and high oil pressure, the storage area oil level can reach higher than the oil pipe. A pass is therefore provided which defines an opening at the top of the storage area connecting the storage area with the sump. If the extreme conditions continue for a sufficient time, the storage oil level can flow freely back to the sump through the pass, which can contact the rotating components.
While these systems are useful for their intended purpose, there is a need in the art for an active control system that minimizes cost and mass and that allows excess hydraulic fluid to be stored out of the sump during normal operating conditions but not during certain other conditions, such as end-of-line testing or transportation of the transmission.